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PostPosted: Sat Feb 06, 2016 2:49 am 
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I had a car that stalled on both fuels, so one naturally blames ignition fault. The usual advice found in various forums is crank angle sensor or DFI module. I changed both of those to no effect. So then changed spark plugs and leads, air filter, PCV, cleaned throttle et cetera. Also unplugged all connectors and cleaned them. Still no joy. I took it to LPG specialist who decided that it must be throttle position sensor or exhaust sensor, since they are used by both systems. And ... kept stalling. So starting thinking: the ECU is flaky.
Then had a look at petrol tank - I found that the vapour separator had failed and petrol was pissing into the charcoal canister. I got air compressor and blew out all the lines, and the canister. It was
good for a day afterwards, bingo. So when purge valve opened, the engine got extra dump of fuel and it must have been too rich to fire.
:shock:


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PostPosted: Sat Feb 06, 2016 8:34 pm 
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No mention of what vehicle this is....

Ignition is a common culprit if a vehicle stalls on both fuels but I wouldn't go as far as 'naturally blames'. It is more likely a reducer diaghpram has gone or LPG tuning (maybe related to state of injectors) is wrong, causing incorrect trims on both fuels.

Unless it's something with an advanced ECU such a (e.g) a supercharged RangeRover, most ECU's don't notice the small difference in air-flow (air with a bit of petrol vapour) that the evap purge system contributes - The purge valve cannot usually be electrically unplugged but the vapour piping can usually be disconnected and plugged to prevent the function.. and all the better for it I reckon! As on hot days a small bit of petrol can be used from the petrol tank by way of burning vapours otherwise... Not possible to do the above with more advanced petrol ECUs because they do notice the difference in air flow and will throw an OBD code if no difference in flow is detected when the purge valve is supposed to be in operation.

Even if the charocal filter is defective, still surprising you're getting liquid petrol entering via the purge valve - tank take-off before purge valve will be at the highest point of the petrol tank anyway? Charcoal mostly there to prevent a flame-front from a backfire reaching the petrol tank? Has the pet tank recently been filled to the top / is this a 4x4 that's been used on steep slopes?

The LPG techs didn't notice this fault? I'm thinking they could have leaned LPG mixture off as part of fault diagnostics / TPS likely only used for the main-part for acceleration enrichment, it's most important function being to ID closed throttle position (to envoke ICV/idle throttle operation).. TPS is easily checked with a multimeter / Regards exhaust sensors - idle would prob be reasonable on petrol even with maxed out fuel trims..

Simon

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PostPosted: Sun Feb 07, 2016 12:19 am 
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Vehicle is a Holden Commodore. The purge valve opens when the engine is warmed up, and throttle position is moderate, i.e. when cruising. Older models purged simply on manifold pressure. The stalling stops when the purge hoses are disconnected or blocked, so this is definitely the cause. It did not stall when idling or under load (hill climbing).
The tank has 2 vapour separators at high points on opposite ends of the tank. I can't find out how they work - maybe they are just float valves. Perhaps one is stuck and when cornering a few drops sloshes out into the the breather hose. So a slug of petrol forms and expanding gas then pushes it into the canister.
The LPG service bloke did say it was running slightly rich on LPG, but the LPG controller only uses one sensor and he did not have a scan tool compatible with my petrol ECU to read both sensors. He was speculating there was an intermittent fault like a solder joint that caused something to drop out.


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PostPosted: Sun Feb 07, 2016 12:23 am 
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I'll hazard a guess it's a Jeep from his username Simon and totally agree with the probably causes you suggested. The thing that concerns me most is how the charcoal chamber ended up full of liquid petrol - this shouldn't happen at all unless somehow the tank is pressurising which would suggest the breather/over-pressure vent in the fuel cap is blocked and/or malfunctioning.

I suspect the charcoal canister is a symptom rather than the cause of this problem and it would be a good idea to renew the petrol filler cap.

Just seen the OPs newest post - my guess of a Jeep was wrong! :oops: :lol: Still think a petrol cap might cure this though as it sounds as if something is pushing petrol quite a way up the vapour separator tubes.

***** EDIT *****

The Holden Commodore is an Australian car - is it still in it's home country? If so it's summer there now so that would reinforce my excess pressure theory a bit.

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PostPosted: Sun Feb 07, 2016 8:59 pm 
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Seems it's already known how to effect a cure with no ill effect (except the environmental impact of venting HC's to atmosphere when refuelling with petrol - purge valve is pretty much there just to prevent pressurised fumes escaping when you remove the petrol filler cap)...

Disconnect the purge valve piping and plug both ends... A lot easier than removing the petrol tank and of no detriment... Running on LPG the vehicle is far better for the environment anyway and the petrol tank cap won't be removed anything like as often.

Simon

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Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
Mid Yorkshire
2 miles A1, 8 miles M62,
http://www.Lpgc.co.uk
Twitter https://twitter.com/AutogasSimon
07816237240


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